Various membrane processes may generally be applied for water treatment or industrial treatment applications. The cost effectiveness of these processes may vary depending on the specific application(s), site considerations, such as energy availability, capital costs, competitive processes and the combination of the membrane properties (such as flux, separation characteristics, performance stability, and fouling resistance as well as the ability to be cleaned when fouled). For water treatment and desalination applications, for example, to-date commonly used membranes are reverse osmosis (RO) and/or nanofiltration membranes (NF) membranes based on polyamide composites. These membranes may generally be considered to have high flux and rejection characteristics, even though from the stability and chemical resistance point there is still a need for improvement. Other NF and RO membranes, such as polyvinyl alcohol and sulfonated engineering plastics membranes, may generally be considered to have better chemical resistance but suffer from flux/selectivity characteristics worse than the polyamide composites membranes. It appears however that for many water applications chemical stability as well as flux and rejection may be considerably important. Therefore the development and manufacture of membranes having the flux/selectivity/rejection properties similar to (or better than) that of the polyamide membranes but with a chemical stability similar to (or better than) the chemical stability of sulfonated polyether sulfones is highly important and may considerably lower the cost of water treatment.
In desalination of water streams, especially seawater and brackish water high rejections without loss of flux would decrease the coast of the final water product. Thus RO membranes with higher salt rejections are very important. There are also many applications in water purifications where one would like to also remove boron or nitrates. Existing RO and NF membranes still do not have the long-term rejections to these components and any membrane or modification that can improve the rejection of NF and RO membranes to these solutes is also important. Higher rejections for example for sea water desalination, or boron removal or nitrate removal to name a few applications.
Other important membrane related applications include electro-membrane processes (Electrodialysis, ED) and Donnan Dialysis (DD). ED and DD processes need to be improved with respect to selectivity in order to reduce the cost of the process.
Existing membranes, such as composite membranes and more specifically, thin film composite membranes, suffer from many disadvantages. For example, coating of a support layer with a discriminating layer may result in variations of thickness of coating material. In areas where no coating is present or where the coating is below a certain thickness the membrane may be exceedingly porous and therefore may not have sufficient separation capability. Where the coating exceeds the desired thickness, the flux may be affected. In membranes where the discriminating layer is prepared separately, the discriminating layer may at least partially separate from the support layer.
There are ionic polymers such as polyelectrolytes and ionomeric materials which offer good potential for making, for example, highly stable selective NF and RO membranes if they can be prepared in a given morphology and fixed into this morphology by crosslinking. A major problem in using such ionic polymers effectively has been the difficulty of first forming a given film or barrier with the required physico-chemical and morphological structures, and then fixing these structures by crosslinking the structures into a given fixed morphology, and the formation of defect free films especially very thin films of less than a micron.
Among the existing membrane related technologies that are commonly applied are the following:
1) Thin film composite or coated membrane having a porous polymeric substrate with one or more microporous layers to which a thin film or coating comprising a sulfonated polyarylether is attached substantively to provide an oxidatively stable, thin hydrophilic film or coating layer. This may be suitable for reverse osmosis, ultrafiltration and microfiltration applications. The sulfonated polymer is applied by coating directly onto the porous support and gives RO membranes. In general the problem with thin selective layers especially for RO and NF based on ionomers and polyclectrolytes is the continuous swelling and change in performance over time and/or in changes in the ionic strength of the solution in systems, which are not crosslinked. Chemically stable crosslinking in chemically stable ionomeric materials is however difficult to achieve.
It is possible in cases like sulfonated polysulfone and polyether sulfone and other sulfonated engineering plastic composites to tailor the performance of composite RO membranes by optimizing the degree of sulfonation of the polyaryl ether thin film polymer to narrow ranges of ion exchange capacity (IEC). As high IEC values needed for high flux give relatively low rejections due to swelling this approach is still not sufficient to achieve high rejecting RO membranes with high flux. This optimum is still also not sufficient to give NF membranes with a combination of high flux and high rejection to organics.
RO membranes with high rejections and fluxes with an order of magnitude better chlorine or chlorine based oxidants or oxidant stability better than polyamide membranes have yet to be developed, particularly for improving the economy of water production NF membranes with a combination of stable high flux, high organic rejections, and chlorine resistance are still lacking.
2) Crosslinking of polymers is used in membrane technology to prepare selective layers of composite membranes and specialized porous supports for composite membranes, for UF and NF integrally skinned membranes made by phase inversion, as self standing films for ED and fuel cells as well as other energy conversion systems (for example, supercapacitors). For example, a method of crosslinking ion-conducting polymers through the sulfonic acid groups to form sulfone crosslinks between polymer chains is described. This method entails sulfonating the polymer (for example, polyetheretherketone (PEEK) using concentrated sulfuric acid, casting of a film, then heating the film which causes the crosslinking to occur. After the crosslinking about many of the sulfonic acid groups in the total solution had been converted to sulfone groups by the cross-linking process. Enough sulfonic groups remain however, for sufficient proton conductivity. This method is often not reproducible in general, works with only certain ionomers such as PEEK, and needs an extensive heating step of hours to days which is often not practical in making composites in general or more specifically of a thin layer composites on UF supports where some of the components may not have the necessary heat stability.
3) Crosslinking of derivatives of 2,6 dimethy phenylene oxide (PPO), especially bromomethylated and sulfonated has been studied. Blend membranes based on sulfonated PPO for polymer electrochemical cells, for example sulfonated PPO, in the hydrogen form readily undergoes crosslinking upon heating under acid conditions and this has been used to stabilize ion exchange membranes of sulfonated PPO in PVDF (polyvinylidene fluoride) mixtures for Fuel cell (FC) and energy conversion devices. These crosslinked membranes, however, do not have good free radical stability and make poor selective RO and NF membranes. The crosslinking step of heating the sulfonic acid form of the PPO results in a loss of sulfonic groups which may change the selectivity and flux properties in an unpredictable way since the SPPO is the main ionomer which confers the selectivity and flux properties in the membrane.
4) Other methods of crosslinking of ion conducting polymers for fuel cells based on sulfonic or phosphonic or carboxylated polymers also exist, for example, composite solid polymer electrolyte membranes (SPEMs) which include a porous polymer substrate interpenetrated with an ion-conducting material. The crosslinking methods described for this method are essentially the methods described above, which have the limitations already described and crosslinking methods, that involve amides and imides crosslinks, which lack oxidant and chemical stability.
5) Sulfinic acid and salts (Li, Na, K, Rb, Cs or other mono- or di-valent metal cations) chemistry has been recently used to produce crosslinked ion exchange membranes based on engineering plastics as well as other polymers. These applications are primarily discussed for ion exchange membranes for fuel cells but have also been proposed for use also in membrane processes in general, such as membrane separation operations, preferably in the context of gas separation, pervaporation, perstraction, reverse osmosis, nanofiltration, electrodialysis, and diffusion dialysis. Different ways discussed of forming crosslinked ion exchange polymers with sulfinc groups are discussed. The sulfuric groups can be reacted with haloalkylated crosslinkers to form alkyl aromatic sulfones crosslinks, or the alkyl groups can be reacted under conditions were the sulfinic groups can disproportinate with each other to form sulfinates (—S(S═O)2S—.
The above approaches of sulfonic polymers require relatively difficult synthesis which would increase the cost of FC and ED membranes. The use of aprotic solvents would make it difficult to produce composite RO and NF membranes by coating the solutions of the precursor polymers of the selective barriers in these solvents on most current UF supports as most supports are not stable to such solvents. Extensive heating under vacuum is also detrimental to facilitate membrane production procedures. The generation of a high concentration of sulfone crosslinks increases hydrophobicity to a great extend, which may limit flux that is important for NF and RO membranes. Even though in some approaches starting with a large number of sulfonic acid groups allow sacrifice of sulfonic acid groups for appropriate cross-linking this is difficult to control.
6) Diazonium crosslinking reactions are also known in preparing membranes by reactions of cationic groups (sulfonium, quaternary ammonium, phosphonium, pyridinium, thiazolinium, imidazolinium and diazonium) with nucleophilic groups. In these processes there is a use of polysulfone (a polyaromatic condensate an engineering plastic), which can be chloromethylated and converted to a water-compatible trimethylammonium hydroxide derivative by reaction with trimethyl amine followed by ion exchange. This product is reacted with p,p′-dimercaptodiphenyl to obtain a crosslinked product, wherein the bond is through the methyl group, which may cause stability related problems.
Diazonium salts were also used in the modification of polyamide membranes.
There is a need in the art for improved and cost effective membranes that may be applied, for example, in any of the fields disclosed herein.